Optical systems have many applications, including communication and materials processing. Such optical systems often employ lasers, for example, fiber lasers, disk lasers, diode lasers, diode-pumped solid state lasers, and lamp-pumped solid state lasers. In these systems, optical power is often delivered by an optical fiber.
A saturable absorber is a device with optical loss, typically, but not always, through absorption, that decreases with increasing optical power or energy. Saturable absorbers find many applications in a variety of optical systems. For example, a saturable absorber may be employed for optical pulse generation in passive laser Q-switching or mode-locking applications. As another example, a saturable absorber may be employed outside of a laser cavity for optical pulse shape modification in non-linear filtering applications. A saturable absorber may also be employed outside of a laser cavity for a wide variety of other optical signal processing applications.
Common absorption-based saturable absorbers include doped crystals (e.g., Cr4+:YAG and other Cr-doped crystals, V3+:YAG, Co2+:MgAl2O4), semiconductor saturable absorber mirrors (SESAMs), and quantum dots (e.g., PbS) in a glass host. Non-absorption-based (i.e. artificial) saturable absorbers employ various mechanisms including reflectance modulation, Kerr lensing, and other non-linear effects. Most of these technologies require the use of free-space beams and bulk optical components, precluding their use in all-fiber systems. Conventional saturable absorbers therefore come with an associated cost, greater optical system complexity, inherent optical losses, and/or reliability and robustness constraints. These constraints may, in turn, limit their applicability, performance, and practicality.
Important properties of saturable absorbers include the saturation energy or fluence, the modulation depth, the recovery time, the non-saturable loss, and the wavelength response. In most conventional saturable absorbers, these properties or characteristics are determined by the material system and can be varied over only a limited range, if at all. A saturable absorber's performance characteristics or properties are thus, in general, not optimized, or tunable, for any given application. Furthermore, there are a number of applications where a suitable saturable absorber simply does not exist.
Saturable absorbers that can remove one or more of the constraints associated with conventional technologies would be therefore advantageous.